Abstract: In order to supply the navigation service with high accuracy for the diver in underwater actions,a scheme of underwater GPS system which takes the active sonobuoy as a communication beacon,is proposed.The principle of target positioning is based on the measurements of sound propagation delay between the target and beacons,and the algorithm for solving the nonlinear equations of target's position is also introduced.Thereafter,the Kalman filter and the extended Kalman filter are designed for improving the accuracy of position estimation,and their performance is demonstrated via numerical simulations.The results show that it is necessary to integrate the smoothing techniques such as the Kalman filtering into the positioning solver to improve the navigation accuracy.